The present invention relates to an improvement in machines for rotational moulding of plastic products, more particular a device for removal of moulded articles from rotational moulding machines.
The invention shall in the present specification be described and illustrated in connection with the production of closed hollow bodies, for instance balls, fenders, buoys, etc. It shall, however, be understood that the invention can be adapted for use in connection with machines for moulding of other kinds of products, for instance containers, toys, etc. There are known several types of machines for rotational moulding of hollow bodies in plastics. These have that feature in common that the mould -- or moulds, during the moulding process rotate simultaneously in two planes, preferably two planes extending normal to each other. A plastic charge in the mould will, during the movement of the mould, in consequence of its gravity, seek at any time the lowermost part of the mould surface whereby the charge is spread uniformly around the interior surface of the mould. By heating the mould the charge will thus finally form an even layer on the mould surface, and the mould can be opened in order to remove the product. Centrifugal forces may, but will normally not, influence the moulding process.
The plastic charge, for instance PVC or polyethylene, constitutes normally a vinyl plastisol composition, a polyethylene powder or possibly a so-called smelt. The mould must usually be heated during the moulding process. In order to heat the moulds there can be used heat ovens with the circulation of hot air. There are also known machines wherein the moulds are encompassed by a mantle for circulation of a hot fluid, for instance hot oil. The applicants have developed a rotational moulding machine wherein for the heating of the moulds are utilized special gas burners mounted on the frame supporting the moulds such that the burner participates in one of the rotational movements of the mould. In a preferred embodiment the mould or moulds are then subjected to a rotational movement in one plane and a tilting movement in the other, and the gas burners participate in the latter movement.
Compared with several other plastic moulding processes, for instance injection moulding, machinery and equipment for carrying out the rotational moulding of hollow bodies have hitherto been hampered by the shortcoming that the moulding process involves one or more steps including manual labour. The applicants have, developed rotational moulding machines which to a large degree operate more or less automatically or without the need for manual labour. Thus, the charging of the moulds prior to the moulding, the closing of the moulds, the double rotational casting process, the heating and possibly the cooling of the moulds, and finally the opening of the moulds are carried out automatically, and in accordance with an accurately stipulated time controlled operational cycle. In the final operational step in the production, more particularly release, removal of the readily moulded products from the moulds in the machines, have, however, hitherto included more or less manual work.
The principle object of the present invention has therefore been to provide mechanical equipment for releasing, firm holding and removal of moulded products from the rotational moulding machine.
It is well known that in the art of rotational moulding of hollow bodies in closed moulds it is difficult, if not to say impossible, accurately to predict how the product will look and conduct itself when the two halves of the mould are moved from each other such that the mould is opened. In the cases where the moulding parts have different shape and wherein for example casting cores or the like are located in the one mould part, one can to a certain degree predict that the moulded product then will be initially released from the mould part not having such cores, if such are not released or disconnected from the inside of the mould, but otherwise it is among other problems difficult to predict to what extent the moulded product will adhere to the mould surface. Even in those cases where the one mould part is so shaped that the moulded product normally will be attached to this part when the mould parts are moved from each other, it may incidentally occur that the product is adhered to the other mould part, such that one must utilize special means for releasing and removal of the product from the mould parts. Furthermore, one faces the situation where the product tears during such operation. In cases wherein the mould parts are identical, the moulded product will rather arbitrarily be positioned in the one or in the other mould part, and the product must then more or less manually be released from that mould part where it is positioned and thereafter be removed from the machine. In any case this operation demands that a machine operator during the moulding process be present when the moulds are opened, such that he, if necessary, is ready to release the product from the one or from the other of the mould parts and thereafter remove the product from the machine. If the moulding machine is of the kind including a plurality of side by side located moulds, this work obviously can be quite cumbersome and time consuming, since as mentioned it is frequently not possible to predict whether the product is located in the one or in the other mould part. Attempts have been carried out to develop mechanical means for doing this removal work, but hitherto no such means have worked satisfactorily. This is probably due to several factors. One factor is that products such as moulded hollow bodies of plastics present a comparatively soft and slippery surface only poorly adapted for being handled by mechanical means. The interior space in the hollow body has usually during the removal from the mould a pressure equal to or less than the surrounding atmospheric pressure, and the hollow body will consequently easily collaps and be inflated and will in any case present an evading or yielding surface, which surface furthermore may present a sloped surface as against a grasping means. A special problem is caused by the fact that it is difficult, if not to say impossible, to predict in which mould parts the product in fact is positioned subsequent to the mould parts having been moved away from each other. A further problem is that mechanical grasping means as such easily may damage the product particularly because these in many cases have not yet been thoroughly hardened and/or cooled down.